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Wednesday, 21st October 2009

Fire danger warnings in Hong Kong

From time to time we receive queries from members of the public and media about the criteria used in warning fire danger. These queries often relate to the issuing of fire danger warnings when the weather conditions may not be conducive to a high fire risk, for instance, when the relative humidity is high, or when rain is falling or on the way.

Several weather factors are considered in the issuing of fire danger warnings, namely relative humidity, wind strength and the chance of rain. Another factor is the state of grass, i.e. the degree of grass curing (dryness). Information on grass curing is provided by the Agriculture, Fisheries and Conservation Department.

As one can easily imagine, low humidity, high wind and little or no rain, coupled with dry vegetation, would be conducive to a high fire risk. Not apparent, however, is the effect of public holidays on the fire danger. I would like to present some preliminary results of a study made by my colleagues.

Daily mean number of hill fires, 2002 -2007
Figure 1

Figure 1 presents the number of hill fires per day, separately for weekdays and for holidays, averaged for each month over the period 2002-2007. It can be seen that hill fires on holidays are consistently higher that those on weekdays.

We can further look into the effect the Ching Ming (around spring time) and Chung Yeung (autumn time) festivals may have on hill fires. Both are public holidays in Hong Kong where Chinese people traditionally go to the hills or cemeteries and pay tribute to their ancestors.
Number of hill fire from 7 days before to 7 days after Ching Ming, 2002 to 2007

Number of hill fire from 7 days before to 7 days after Chung Yeung, 2002 to 2007
Figure 2      Number of hill fires around the times of Ching Ming and Chung Yeung, from 2002 to 2007.
The red bars represent figures on the day of Ching Ming and Chung Yeung. Daily figures for the
7 days prior to and 7 days after the festival event are plotted to its left and right respectively.

Figure 2 presents hill fire figures around the time of Ching Ming (top) and of Chung Yeung (bottom) for the period 2002-2007.

The numbers speak for themselves. Taking Figure 1 and 2 together, one can readily see that hill fire figures for Ching Ming and Chung Yeung far outstrip those for ordinary days and other holidays. Statistics behind Figure 2 also indicate that hill fires during Chung Yeung nearly double those of Ching Ming over the 6-year period, probably a result of lower humidity and drier vegetation.

One can also notice that fewer hill fires were reported for the Ching Ming of 2003 (Figure 2(a)), probably because people stayed home during the SARS epidemic at the time. Another low in the number of hill fires occurred in 2007, and this could be attributed to an awareness campaign for 'zero hill fire' conducted early that year. However, memory was short and hill fires returned with a vengeance in Chung Yeung (Figure 2(b)) the same year. Leaving fires behind in the countryside is the main culprit. The purpose of fire danger warnings is to remind people not to do just that.

As you can see, while meteorological assessment of the fire risk must be based on science (for example, weather measurements), operation of the fire danger warnings in order to save lives and rescue our precious trees would not be effective without taking account of societal factors.

B.Y. Lee

Last revision date: <17 Jan 2013>